1. Field of the Invention
The present invention relates to a liquid crystal display device of a normally black mode, and specifically to a liquid crystal display device preferably usable for displays requiring a wide viewing angle, such as TVs and monitors for personal computers including personal computers used for CAD. In this specification, the liquid crystal display device will be referred to as the xe2x80x9cLCD devicexe2x80x9d.
2. Description of the Related Art
As infrastructures for information technology are more and more developed, TVs, personal computer monitors, and other devices usable as a terminal for providing video and audio information are now required to be increasingly reduced in size. Especially, more and more devices such as small- or middle-sized TVs and personal computer monitors adopt LCD devices in order to meet the demands of society to save space and power.
For this purpose, twisted nematic mode LCD devices of an active driving system and super twisted nematic mode LCD devices of a passive driving system have been developed and widely used.
However, application of twisted nematic mode or super twisted nematic mode liquid crystal materials to a liquid crystal panel which is often used for a small- or middle-sized TV or a personal computer monitor, results in a smaller viewing angle. A liquid crystal panel having an unsatisfactorily small viewing angle has problems in that, for example, (i) one same color appears different at two ends of a screen and (ii) images appear differently from different viewing angles. For example, when a plurality of people watch the image, the image appears differently to each individual. The image also appears differently when the viewer sits up straight from when the viewer sits in a relaxed manner. Accordingly, the twisted nematic mode or super twisted nematic mode liquid crystal materials cannot be applied to a TV.
The twisted nematic mode or super twisted nematic mode liquid crystal materials have another problem in that the color tone changes area by area in a large screen. This problem prevents positive use of liquid crystal materials for personal computer monitors.
In order to solve these problems, LCD systems such as, for example, a multiple domain twisted nematic system (including a plurality of regions having different alignment directions of liquid crystal molecules) (Japanese Laid-Open Publication No. 5-107544), an ASM display system (Japanese Laid-Open Publication No. 6-301015), an MVA display system (Japanese Laid-Open Publication No. 8-43825), and an IPS display system (Japanese Laid-Open Publication No. 7-36058) have been proposed. However, none of these systems provides satisfactory display characteristics and involve the problem of higher cost.
The liquid crystal materials have birefringence anisotropy thereof change in accordance with the temperature, which involves an undesirable possibility of reducing the contrast of an image displayed by the LCD device.
A liquid crystal display device according to the present invention includes a pair of polarizing plates; a liquid crystal panel interposed between the pair of polarizing plates, the liquid crystal panel including a liquid crystal layer having liquid crystal molecules, the liquid crystal layer including a pair of regions corresponding to each of pixels, alignment directions of the liquid crystal molecules in the pair of regions being different by about 180 degrees from each other, the liquid crystal panel having a first slow axis; a first phase plate provided between the liquid crystal panel and one of the polarizing plates, the first phase plate having a second slow axis which is horizontal to a surface thereof and substantially perpendicular to the first slow axis of the liquid crystal panel; and a second phase plate provided between the liquid crystal panel and the one of the polarizing plates, the second phase plate having a third slow axis which is substantially perpendicular to both of the first slow axis of the liquid crystal panel and the second slow axis of the first phase plate. The liquid crystal material has a dependency of birefringence anisotropy (xcex94n) from 20xc2x0 C. to 60xc2x0 C. ([xcex94n(20xc2x0 C.)xe2x88x92xcex94n(60xc2x0 C.)]/40) of 0.00026 or less.
In one embodiment of the invention, the second phase plate further has a fourth slow axis which is parallel to a surface thereof.
In one embodiment of the invention, the first slow axis of the liquid crystal panel is at an angle of about 45 degrees with respect to a polarizing axes of the polarizing plates when no electric field is applied to the liquid crystal panel.
In one embodiment of the invention, the liquid crystal material has a dependency of birefringence anisotropy (xcex94n) on the temperature from 20xc2x0 C. to 60xc2x0 C. ([xcex94n(20xc2x0 C.)xe2x88x92xcex94n(60xc2x0 C.)]/40) of 0.00014 or more.
In one embodiment of the invention, the liquid crystal material has a dependency of birefringence anisotropy (xcex94n) on the temperature from 20xc2x0 C. to 60xc2x0 C. ([xcex94n(20xc2x0 C.)xe2x88x92xcex94n(60xc2x0 C.)]/40) of 0.0002 or more.
In one embodiment of the invention, the liquid crystal display device further includes a third phase plate having a fifth slow axis which is substantially perpendicular to the first slow axis of the liquid crystal panel and parallel to the second slow axis of the first phase plate; and a fourth phase plate having a sixth slow axis which is substantially perpendicular to both of the first slow axis of the liquid crystal and the fifth slow axis of the third phase plate. The third phase plate is provided between the liquid crystal panel and the other polarizing plate so as to be symmetrical to the first phase plate with respect to the liquid crystal panel, and the fourth phase plate is provided between the liquid crystal panel and the other polarizing plate so as to be symmetrical to the second phase plate with respect to the liquid crystal panel.
In one embodiment of the invention, the first phase plate and the third phase plate have a substantially equal birefringence to each other, and the second phase plate and the fourth phase plate have a substantially equal birefringence to each other.
Thus, the invention described herein makes possible the advantages of providing (1) a liquid crystal display device having a satisfactorily wide viewing angle at lower cost, and (2) a liquid crystal display device for alleviating the reduction of contrast of images despite a temperature change.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.